A faucet aerator is the small screened tip that screws onto the end of a faucet spout. It mixes air into the water stream, which produces a steady, splash-free flow while using less water. Aerators are a simple, cheap way to cut water use, and a clogged one is a common cause of weak flow at a single faucet.
A faucet aerator is the little fitting on the very tip of a faucet, the part the water passes through last before it reaches your hands. Unscrew the end of almost any modern kitchen or bathroom faucet and you will find one: a small housing holding a fine screen or a stack of tiny discs. Its name comes from its main trick, which is mixing air into the water as it leaves the spout.
That air does two useful things. First, it shapes the water into a soft, even stream instead of a hard single jet, which greatly reduces splashing in the sink. Second, and more importantly, it lets the faucet feel full and forceful while actually using less water. The aerator restricts how much water can pass and blends in air to make up the apparent volume, so a low-flow faucet still feels strong. This is why aerators are one of the cheapest and easiest ways to save water. The EPA notes that efficient bathroom faucets and aerators can cut a sink's water use meaningfully without a noticeable drop in performance.
Aerators are rated by their flow rate, measured in gallons per minute, and low-flow fixture rules set limits in some areas. A standard older aerator might pass a higher flow, while a water-saving model passes less. Swapping an old aerator for a lower-flow one is a five-minute, low-cost upgrade that reduces both water and, at hot taps, water-heating energy.
The aerator is also a common troublemaker, in a good way. Because that screen is the last thing the water passes through, it catches grit, rust flakes, and mineral scale that break loose upstream in the pipes. Over time this debris clogs the screen and chokes the flow. For example, if the water at one particular faucet has slowed to a trickle while every other tap is fine, a clogged aerator is the most likely cause, and the fix is often as simple as unscrewing it and rinsing the screen. See low water flow from one faucet for how to clear it, and low water pressure at one fixture if the whole faucet is weak.